1. Field of the Invention
This invention relates to mercapto heterocyclic toning agents in photothermographic compositions and elements comprising silver salts of certan heterocyclic thione compounds and to photothermographic processes for preparing developed images with them. In another aspect the invention relates to such photothermographic elements, compositions and processes comprising photographic silver halide in association with the silver salt of certain heterocyclic thiones. A further aspect of the invention relates to thermographic elements, compositions and processes wherein the described toning agent provides improved tone.
2. Description of the State of the Art
Photothermographic materials comprising a photosensitive component such as photographic silver halide with a silver salt of certain heterocyclic thione compounds are described, for example, in U.S. Pat. No. 3,785,830 issued Jan. 15, 1974. Such photothermographic elements comprise (a) a photosensitive component consisting essentially of photographic silver halide, such as photographic silver iodide, (b) a reducing agent, and (c) a silver salt of a thione compound, such as a silver salt of a thiazoline-2-thione, benzothiazoline-2-thione, imidazoline-2-thione or oxazoline-2-thione. These photothermographic elements and compositions can provide a developed image which is stable in the absence of a separate stabilizing compound. The problem encountered with these photothermographic elements and compositions is that the developed image resulting from them does not have the desired black tone. Also, the maximum density is often less than desired.
None of the art describing various photothermographic elements and compositions has provided a useful answer to this tone problem. Typical photothermographic elements are described, for example, in U.S. Pat. No. 3,152,904 of Shepard et al, issued Oct. 13, 1964; U.S. Pat. No. 3,392,020 of Yutzy et al, issued July 9, 1968; U.S. Pat. No. 3,457,075 of Morgan et al, issued July 22, 1969 and U.S. Pat. No. 3,672,904 of deMauriac, issued June 27, 1972. Toning agents known in these photothermographic materials and toning agents known in thermographic materials have not, as a class, provided a satisfactory solution to the described tone problem.
Heterocyclic mercapto compounds, as a class, do not provide a useful solution to this problem. Heterocyclic mercapto compounds have been used for various purposes in photographic elements, such as photothermographic elements. Such heterocyclic mercapto compounds in photographic and photothermographic elements are described, for example, in U.S. Pat. No. 3,617,289 of Ohkubo et al, issued Nov. 2, 1971; U.S. Pat. No. 3,396,017 of Bacon et al, issued Aug. 6, 1968; U.S. Pat. No. 3,017,270 of Tregillus et al, issued Jan. 16, 1962; U.S. Pat. No. 2,573,027 of Kendall et al, issued Oct. 30, 1951; U.S. Pat. No. 2,590,775 of Kendall et al, issued Mar. 25, 1952; U.S. Pat. No. 2,432,506 of Chilton et al, issued Dec. 16, 1947; Canadian Pat. No. 905,191 issued July 18, 1972 and Belgian Pat. No. 768,071 issued July 30, 1971. The fact that heterocyclic mercapto compounds as a class do not provide the desired toning results is demonstrated in comparative examples in following Table II.
There has been a need to provide photothermographic elements, compositions and processes which produce improved tone in a developed image wherein the photothermographic elements and compositions comprise photographic silver halide in association with a silver salt of certain heterocyclic thiones, as described herein, and an organic reducing agent. There has also been a need to provide thermographic elements, compositions and processes which provide improved tone wherein the thermographic composition or element comprises the described silver salt of certain thione compounds with an organic reducing agent. There has also been a need to provide developed images in such compositions and elements which are stable and do not require a separate stabilizer or stabilizer precursor component.